Conventional wireless communication standards, such as those for Bluetooth and WiFi systems (e.g., 802.11xx, 2.4 GHz or 5 GHz bands, etc.) allow for a receiver to measure signal strength from an external RF transmitting source, such as smartphone or other wireless device, for example. One measure of signal strength is received signal strength indication (RSSI). RSSI may be regarded as an indication of RF power being received by an antenna of the receiving wireless device. High RSSI values are indicative of a strong signal and low RSSI values are indicative of a weak signal. In that the RSSI is a relative measure of received signal strength, the units of measure for RSSI may be in arbitrary units. For example, in one application RSSI may be assigned arbitrary units of 0 to 100 or 0 to some maximum value of RSSI. Therefore, units of actual measured power, such as mW or dBm need not be used and may not be helpful in determining relative strength or weakness of received signal strength in a wireless environment.
In some applications it is desirable to use RSSI to estimate distance between the transmitting device and the receiving device. For example, if the transmitting device and receiving device are approximately 10 cm away from each other, then the RSSI should be stronger than when they are 1 meter away from each other. However, there are known difficulties in using RSSI readings for accurate distance measurements due to many factors including but not limited to: (a) multipath effects caused by RF signal reflection off surrounding objects such as walls, moving objects, and stationary objects; (b) differences in antenna radiation patterns and polarization patterns of the transmitting and receiving antennas; and (c) RF interference generated by other radiators of RF energy in the wireless environment of the receiver that is attempting to measure the RSSI of a specific transmitter; just to name a few. Generally, close distance RSSI measurements may be made with a higher accuracy than long distance measurements due to the inverse square power drop off of the RF signal (i.e., 1/R2) in the far field region and a greater drop off (e.g., greater than 1/R3) in the near field region. Close proximity sensing using RSSI has a statistically higher level of accuracy and a receiving device may infer that it is in close proximity to a transmitting device when both devices are close to one another. However, there remains a small probability that a false alarm may be triggered when the RSSI indicates close proximity when in fact the two devices are not in close proximity to each other.
Thus, there is a need for systems that allow for accurate RF signal detection to be made in close proximity between transmitting and receiving devices without relying solely on RSSI information or that don't use RSSI information for determining proximity.